There is a growing demand for broadband wireless access systems which can deliver the high data rates required for the provision of multi-media services. Such wireless access systems operate within licensed frequency bands. Accordingly, these systems are continually developing to carry more data across the limited frequency band allocated to them. Pressure for this development is two fold. Firstly, there is increased demand for multi-media services from subscribers to the system. Secondly, revenue for the network operator will increase as billing is calculated on a per byte of information delivered basis as opposed to on a timed basis.
The performance of wireless access communication systems is prone to dynamic degradation, ie. time variant degradation, due to changing environmental conditions. Wireless transmissions in the frequency range from 10 to 50 GHz are particularly prone to dynamic degradation resulting from rain and from the growth and movement of foliage located in the path of the transmission. FIG. 1 shows the attenuation of a 30 GHz signal in dB per kilometer due to rainfall against the percentage of time that such rainfall occurs within climate zone ‘F’ which zone covers the UK.
This type of dynamic degradation has been taken account of in existing wireless access systems by designing the systems for operation in worst case environmental conditions. This has been achieved by the use of robust modulation schemes such as QPSK (Quadrature Phase Shift Keying), also known as 4-QAM (Quadrature Amplitude Modulation) which deliver low BERs (bit error rates) of the order of 10−9, ie. one incorrect bit per 109 bits transmitted, in poor environmental conditions. However, designing such systems for worst case environmental conditions in this way results in low rates of data transmission.
As can be seen from FIG. 1, for the majority of time transmission conditions are good. Adaptive modulation techniques have been proposed which enable higher data rates to be achieved by the use of 16-QAM or 64-QAM modulation schemes when the transmission conditions across a wireless link are improved or where the distance over which the link extends is relatively low. In this way the rate of data transmission within a limited frequency band can be improved.
In known cellular wireless access system a frequency plan is implemented over a geographical area covered by the system. The frequency plan allocates channels within the frequency band to localised cells and due to attenuation of a radio signal across the cells, the same channel can be reused within other cells in the frequency plan. The aim is to maximise frequency re-use without causing interference between parts of the frequency plan which use the same channels. Generally, a base station is associated with a cell to transmit radio frequency signals to all end users or CPEs (Customer Premise Equipments) located within the geographical area covered by the cell. The uplink from the CPEs in the cell to the base station may be a common medium access uplink, for example FTDMA (Frequency or Time Division Multiple Access) uplink in which time and carrier slots can in some way be allocated for use by the CPEs to send signals to the base station. The downlink from the base station to the CPEs may be a TDA (Time Division Access) downlink, with time slots over which the base station sends signals to the CPEs.
One known adaptive communication system is disclosed in U.S. Pat. No. 5,940,439 and operates by varying the coding rate, modulation method and the symbol rate responsive to the status of the radio transmission channels or carriers. The system provides improvements in coded operation to take into account changing communication channel conditions. The system determines optimal voice coding rates, coding strategies and modulation for optimum voice quality and intelligibility. Because three variables are altered, ie. modulation scheme, symbol rate and coding rate, there is no unique solution to the choice of these variables for a set channel status. The bandwidth of the channels used in this system will have to be set to accommodate the maximum symbol rate that can be selected. Thus, when the optimum symbol rate is less than this maximum symbol rate bandwidth will be wasted. Accordingly, the system described in U.S. Pat. No. 5,940,439 is not efficient in its use of bandwidth. Also, U.S. Pat. No. 5,940,439 does not provide an algorithm for determining the three variables.
Another approach to optimising the use of bandwidth is automatic repeat request (ARQ). In this approach the receiving unit, be it a base station or a CPE, detects which signals sent across the transmission link have been received with errors in them and sends a feedback message to the transmitting unit requesting that the signals which have not been correctly received are sent again. This is an alternative way of increasing or decreasing the amount of information which is sent across the transmission link dependent on environmental conditions. In poor transmission conditions, more data will have to be se-sent and so data rates will be low. In good transmission conditions, less data will have to be re-sent and so data rates will be higher. However, this method has a degree of transmission delay inherent within it which may not be appropriate for all multimedia services. It is also inefficient in terms of the amount of uplink or downlink resource used for services requiring a low bit error rate.
It is also known to use forward error correction FEC) in which a FEC code is added to the data payload of a packet sent across a transmission link. The FEC code is used by the receiving unit to detect and correct errors in the data payload received by the receiving unit, so that they do not have to be re-sent over a transmission link.